With over 1.2 million new cases and over 608,000 deaths annually, colorectal cancer (CRC) is the third most common cancer and the third highest cause of cancer death in the developed world. CRC patients are classified into stage I through IV depending on the extent of their disease. Approximately 25% of CRC patients are diagnosed with localized stage II cancer, amounting to 300,000 new cases in the developed world and 35,000 in the U.S. annually. The standard of care for stage II CRC includes surgical removal of the tumor followed by adjuvant therapy in high risk patients. The current risk stratification approaches, including the standard of care TNM staging method, offer only limited accuracy, as evidenced by the approximately 20-25% of stage II CRC patients who recur with predominantly incurable disease and do not receive adjuvant therapy. Therefore, novel, more accurate approaches to identify high risk patients are urgently needed. Circulating cell-free tumor-derived DNA (ctDNA) is released by tumors and carries tumor-exclusive genetic alterations. Hypothesis: We hypothesize that direct and early detection of minimal residual disease (MRD) using ctDNA will more accurately identify high risk CRC patients than the current approaches that predict recurrence based on analyses of the resected tumors. Preliminary Data: We have pioneered the development of technologies for evaluation of ctDNA, and established ctDNA as an exquisitely specific and sensitive marker for tumor burden and MRD. Most relevant to this proposal, we have demonstrated that >75% of localized CRC release detectable ctDNA and that post-surgery ctDNA levels are prognostic. Specific Aims: In this phase I SBIR, we propose to develop and validate CRCDetect, a molecular test for the detection of MRD using ctDNA in the peripheral blood of stage II CRC patients collected 4-6 weeks after surgery. CRCDetect can identify patients who are not cured by surgery alone, have a high risk of recurrence, and may benefit from adjuvant therapy. In Specific Aims 1 and 2, we will focus on the development and analytical validation of CRCDetect. In Specific Aim 3, we will evaluate the prognostic performance of CRCDetect in a cohort of stage II CRC patients. Overall Impact: Together, these studies will demonstrate the feasibility of using CRCDetect to detect MRD in early stage CRC patients from a simple blood draw after surgery, thereby identifying the stage II CRC patients with a high risk of recurrence and informing whether a patient should receive adjuvant treatment.